There is still no obvious (cost-effective) replacement for high-lead, high melting ( 260 - 320 C) solder alloys It is not possible to adjust (to increase above 260 C) liquidus temperature of any existing Sn- based solder alloys by simple alloying with environmentally friendly and inexpensive elements Therefore, in the quest for (cost-effective) replacements of the high-lead solders, attention has to be turned towards different base metals as well as the exploration of alternative joining techniques !
Liquidus projection of the Zn-Al-Mg system Ternary eutectic at ~ 343 C
TMS 2008 Annual Meeting, New Orleans March 9-13, 2008 “Interfacial behaviour between Bi-Ag Solders and the Ni -substrates” (Hsin-Yi Chuang and Jenn-Ming Song) “Interfacial Reaction and Thermal Fatigue of Zn- 4wt.%Al-1wt.% Cu/Ni Solder Joints” by Y. Takaku, I. Ohnima, Y. Yamada, Y. Yagi, I. Nakagawa, T. Atsumi, K. Ishida
The DSC heating curve of the eutectic Bi-Ag alloy
Solidification microstructure of the Bi-Ag eutectic alloy (BEI)
Solidification microstructure of the Bi-Ag hypo-eutectic alloy (BEI) Ag
Transient Liquid Phase (TLP) Bonding solid solid interlayer(s) The interlayers are designed to form a thin or partial layer of a transient liquid phase (TLP) to facilitate bonding via a brazing-like process in which the liquid disappears isothermally In contrast to conventional brazing, the liquid disappears, and a higher melting point phase is formed at the bonding temperature
Transient Liquid Phase (TLP) Bonding Any system wherein a liquid phase disappears by diffusion, reaction (amalgamation), volatilization, or other processes is a candidate for TLP bonding ! solid T = const liquid solid solid product T = const Diffusion, Reaction solid
The effect of Ni additives in the Cu-substrate on the interfacial reaction with Sn
Concluding Remarks Through the judicious selection of Sn- or Bi-based interlayer between under bump metallization and substrate pad, (cost-effective) Transient Liquid Phase (TLP) Bonding can be achieved at ~ 250-280 C, and the resulting joints are capable of service at elevated temperatures ! Therefore, in the quest for (cost-effective) substitutes for high-lead solders, attention has to be turned towards different base metals as well as the exploration of alternative joining techniques ! It is not possible to adjust (to increase above 260 C) liquidus temperature of any existing Sn-based solder alloys by simple alloying with environmentally friendly and inexpensive elements The TLP Bonding should be taken into further consideration as substitute for the high-lead soldering !